By using Pro/ENGINEER to carry on modleing and stimulating the modle’s moving, it analyses a double crank mechanism and finds its crank angle between extreme positions, then gives a method to prove that double crank mechanism has quick-return characteristics.

This paper gives a precise definition about crank angle between extreme positions for planar four-bar linkage and deduces the range of crank angle between extreme positions of crank-rocker mechanism , slider-crank mechanism and crank shaper mechanism.

?This paper probes the scope of four bar mechanism s nature of rapid return, while according to above, thc conception of the angle between two limit places that have given definition in current various material is given a definition anew.

Referrence[1] and [2] have corrected and redefinited the conventional definition about included angle of extreme positions.

This paper analyzes the limited position angle of plant crank mechanism with quick back characteristics,discusses the relationships between the ratio coefficient of journey to speed and speed and acceleration,and points out the range of ratio coefficient of journey to speed in mechanism.

the limitation has overcome in the application of many existing textbooks between the ratio coefficient of Journey to speed and the limited position angle, and from the engineering reality, Range of ratio coefficient of Journey to spee is made a summary.

The quick return characteristics of planar linkage mechanisms have closely relationship with its crank angle θ between the two limiting positions.